1. Field of the Invention
The present invention relates to a pneumatic fastener driving tool for driving fasteners such as nails, staples or the like into work pieces.
2. Description of the Prior Art
A typical pneumatic fastener driving tool of the type concerned is disclosed in Japanese Patent Laid-open Publication No. 61-117074. The disclosed fastener driving tool includes, as reillustrated here in FIG. 28 of the accompanying drawings, a tool body 1 having a push lever 2 adapted to be forced against a work piece 3 before a trigger 4 is actuated. Upon actuation of the trigger 4, a main valve 5 operates to move a main piston 6 forwardly (downward in the same figure) to thereby drive the leading fastener 7 to the work piece 3. Then an actuator 9 of a feed valve 8 disposed rearwardly (upward in the same figure) of the main valve 5 is pushed by the user's finger to move a feed valve element 10 in a direction to open the feed valve 8 whereupon compressed air stored in an accumulator 11 is allowed to flow through a passage 12 and acts on a feed piston 13 of a fastener supply unit 14, thereby driving the fastener supply unit 14 to feed the next following fastener 7 to a barrel 15 of the fastener driving tool.
Since the actuator 9 of the feed valve 8 is disposed remotely from the trigger 4, it is difficult for the user to operate the thus disposed actuator 9 with a finger of a hand while gripping a handle 16 with the same hand. To feed the fasteners 7 one at a time to the barrel 15, the user must operate the actuator 9 with the other hand. Such fastener feeding operation is tedious and time-consuming and hence considerably lowers the fastener driving efficiency. Furthermore, the conventional fastener driving tool is difficult to operate with a single hand and hence is not suited for a horizontal or an overhead fastener driving work in which one hand of the user is occupied in holding a work piece in a desired position. Thus the working position of the conventional fastener driving tool is substantially limited to a flat position.
The conventional fastener driving tool shown in FIG. 28 has another drawback resulting from the construction and arrangement of the fastener supply unit 14, as described below. The fastener supply unit 14 includes a feed prong 17 connected to the feed piston 13 and driven by the feed piston 13 to reciprocate in a direction perpendicular to the axis of the barrel 15 so as to feed the fasteners 7 one at a time to the barrel 15. The feed piston 13 is normally urged by a return spring 18 in a direction such as to move the feed prong 17 toward the barrel 15. Upon operation of the actuator 9, the fastener supply unit 14 is operative regardless of the movement of the main piston 6. This arrangement is advantageous in that a single fastener can be struck several times by repeating a reciprocating movement of the main piston 6. However, a problem arises when the actuator 9 is inadvertently pushed when a fastener 7 is still present in the barrel 15. In this instance, the feed piston 13 is retracted by the compressed air against the force of the return spring 18 until the feed prong 17 engages the second leading fastener 7. When the actuator 9 is released, the valve element 10 of the feed valve 8 is shifted in a direction to block fluid communication between the accumulator 11 and the feed piston 13 and, at the same time, allow the compressed air to escape from the passage 12 through the feed valve 8 to the atmosphere. The feed prong 17 is urged forwardly by the force of the return spring 18 acting on the feed piston 13, however, advancing movement of the feed prong 17 does not take place because the fastener 7 already existing at an inlet of the barrel 15 prevents the leading fastener 7 from moving into the barrel 15. The inlet of the barrel 15 is cleared out when the fastener 7 loaded therein is driven by the reciprocating movement of the main piston 6 whereupon the leading fastener 7 is automatically loaded in the barrel 15 by the spring-loaded feed prong 17 before the next blow of the main piston 6 is applied to the once-struck fastener 7. With this double loading of the fasteners 7, the second loaded fastener is driven onto the first-loaded fastener 7 In this instance, the second-loaded fastener 7 is likely to fly out from the barrel 15 and may hurt the user. The possibility of such harmful double loading of the fasteners is enhanced when the fastener driving tool is provided with an automatic repeated striking mechanism having a repeat valve which enables automatic repeated reciprocation of the main piston 6 so long as the trigger 4 is actuated.
A typical example of the automatic repeated striking mechanism is disclosed in Japanese Patent Publication No. 57-36114. The disclosed automatic repeated striking mechanism includes, as reillustrated here in FIG. 29, a main valve 20 slidably disposed in a main valve chamber 21. When a trigger (not shown but similar to the trigger 4 shown in FIG. 28) is actuated, compressed air is discharged from the main valve chamber 21 through a first passage 22 to the atmosphere whereupon the main valve 20 is displaced upwardly. With this upward movement of the main valve 20, the compressed air rapidly flows from a striking air chamber or accumulator 24 into an upper piston chamber 23, thereby thrusting a piston 25 downwardly to drive a fastener (not shown) to a work piece. The upward movement of the main valve 20 further causes a changeover valve 26 to shift or change its valve position whereupon the compressed air flows from the accumulator 24 through a second passage 27 into a repeat valve chamber 28 to move a repeat valve 29 downward, thereby interrupting or closing the first passage 22. Then, the compressed air flowing from a third passage 30 into the main valve chamber 21 increases the pressure in the main valve chamber 21 whereupon the main valve 20 is lowered to open a discharge valve 31 to thereby discharge the compressed air from the upper piston chamber 23. Since a lower piston chamber 32 and a return air chamber 33 retain therein the compressed air which is supplied during downward movement of the piston 25, the piston 25 is displaced toward its uppermost position as the compressed air is discharged from the upper piston chamber 23. The downward movement of the main valve 20 causes the changeover valve 26 to shift or change its valve position to discharge the compressed air from the repeat valve chamber 28 through the second passage 27 and through an adjustable throttling valve 34. Thus, the repeat valve 29 moves upwardly to open the first passage 22, thereby discharging the compressed air from the main valve chamber 21. Since the cross-sectional area of the first passage 22 is larger than the cross-sectional area of the third passage 30, the main valve 20 again moves upwardly so that the fastener striking operation by the piston 25 is repeated. Reference numeral 35 denotes a body of the fastener driving tool, 36 a cylinder in which the piston 25 reciprocates, 37 a discharge hole, and 38 an adjustment screw associated with the throttling valve 34 for adjusting the repetition cycle time.
With this construction, while the non-illustrated trigger is being actuated, the main valve 20 and the repeat valve 28 operate alternately to intake the compressed air into the upper piston chamber 23 and subsequently discharge the compressed air from the upper piston chamber 23, so that the reciprocating fastener driving movement of the piston 25 is automatically repeated.
The conventional automatic repeated striking mechanism of the foregoing construction is not satisfactory for the reasons described below. The repeat valve 29 is reciprocated by the compressed air which is introduced into and discharged from the repeat valve chamber 28 in response to reciprocating movement of the main valve 20. Thus, the movement of the repeat valve 29 is not directly related to the movement of the piston 26. Accordingly, when the pressure of the compressed air is relatively low or when the fastener driving tool is operating at a relatively short repetition cycle time, the repeat valve 29 is operated to commence a next fastener striking movement of the piston 25 before the return stroke of the piston 25 is completed. With this incomplete return stroke of the piston 25, a complete driving of the fastener is difficult to achieve. Furthermore, the next fastener cannot be supplied because the fastener supplying operation is timed with the fastener striking movement of the piston 25.
Another drawback associated with the conventional automatic repeated striking mechanism is as follows. The compressed air is discharged from the upper piston chamber 23 as the main valve 20 is lowered. In this instance, the discharge valve 31 is opened before the upper end of the cylinder 36 is closed. Accordingly, an excess amount of compressed air is discharged from the accumulator 24 which is held in communication with the upper piston chamber 23 until the upper end of the cylinder 36 is closed. In order to reduce the amount of discharged compressed air, the main valve 20 must be lowered as fast as possible by, for example, increasing the amount of the compressed air taken into the main valve chamber 21. However, partly because the intake of the compressed air to the main valve chamber 21 is achieved solely through the third passage 30, and partly because the third passage 30 is continuously held in fluid communication with the main valve chamber 21 and the accumulator 24, if the cross-sectional area of the third passage 30 is increased, a large amount of compressed air will be discharged through the third passage 30 during the discharge stroke of the main valve chamber 21 and, therefore, a pressure drop large enough to operate the main valve 20 will not be created in the main valve chamber 21. Thus, the third passage 30 of the conventional automatic repeated striking mechanism cannot be enlarged in its cross section and hence an excessively large amount of compressed air is consumed.